Hi folks,

two weeks ago I get my first prewar tv set. It is a Marconiphone 707. The set was repaired in the 1990s. The bad Emiscope crt with broken filaments was replaced by a regunned 9" crt. Nearly all capacitors were replaced by new ones. The line output tube, KT63, was replaced by an EL36.

When I got it, there was no sound and only a small bright spot on the screen. The rectifier tube, U52, had broken filaments. I replaced it with a 5U4. Now the set displayed a bright raster (see first photo). AM sound, but no TV sound. I had to readjust the oscillator. After that, I got sound and video.

Due to the non-fitting crt which usually operates with more than 10 KV, focus is bad and width is not sufficient. The line oscillator was running with the half of the line frequency with double pictures on the screen (see the second and the third photo).

Meanwhile I have set it back to full line frequency, but with the result of a much smaller picture (see last photo). My next steps will be to replace the EL36 by the original KT63 and to replace the 9" crt by a 7" crt.

Kind regards,
Eckhard

Very nice! Thanks for sharing! :thmbsp:

Way to go, Eckhard! Do you still have the original 7" CRT? As you may already know, there is a French company which is likely going to attempt some pre-war CRT rebuilds. Talk to Jerome Halphen, he knows the details.

Wow...That is just TOO KEWL !! Way to go, Eckhard !! You Da Man ! (grin)

John:

Way to go, Eckhard! Do you still have the original 7" CRT? As you may already know, there is a French company which is likely going to attempt some pre-war CRT rebuilds. Talk to Jerome Halphen, he knows the details.

I don't have the original Emiscope, but I hope that I can get it in the next weeks. I have also read about the crt rebuilding process in Paul Stenning's forum. It is an interesting thing. But before I deliver it to the French company (if I get the Emiscope), I want to wait for the reports about other crt projects.

At first, I want to replace the present crt-construction in the set with a 7" crt, and I want to solve the focus and width problems.

- Eckhard

Meanwhile I have got a replacement crt from Darius (oldeurope). It is a radar crt, a 7BP7. The crt fits exactly in the Marconi 707. I had to remove a small amount of the cardboard from the center of the yoke, then I could slip the yokes over the neck of the 7BP7.

One remaining fault is in the line output transformer. I do not get sufficient width and linearity. I have to look for a replacement LOT or have to rewind it.

- Eckhard

I doubt that set will run right or even acceptably on anything other than a 405 line signal from a standards converter.

I doubt that set will run right or even acceptably on anything other than a 405 line signal from a standards converter.

This set is running all the time with a 405 line signal from a standard converter.

Aha, I made one of those fatal assumptions. :) Only because it appeared to have the same foldover and size issues as Hugo Holden's HMV 904 set did when being fed a 625-line signal:
http://www.earlytelevision.org/images/p8110038.jpg
But not really the same because in this pic the tube is upside down, so the foldover is actually on the right.

Hi David,

the foldover seems to be a general problem of these sets.

Tonight I have removed the lline output transformer and tested various other transformers. The best result I got with a flyback transformer from a hybrid color tv set. The width was much wider than the screen width. And I had some arcing in the output tube. So, I replaced the KT63 with an EL36. I added 800 Ohms in the B+ voltage, and I set picture width at minimum. The feedback cap C85 with 230 pF was replaced by 120 pF. With that I got a reasonable result.

This is not the final solution. But this shows the way to go.

Note, that no centering adjustments were done.

Regards,
Eckhard

One remaining fault is in the line output transformer. I do not get sufficient width and linearity. I have to look for a replacement LOPT or have to rewind it.

Hi,

the lopt was actually bad. Ed Dinning in UK has rewound the lopt. Today I got the rewound lopt, and tonight I have installed the rewound lopt. The tv set has full line width without any further complications. Ed did a great job. I am so happy that the lopt is working properly now.

Some work on the focus coil is on my to-do list.

Kind regards,
Eckhard

Hi,

here are some photos which show the progress:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassistotal.jpg

Chassis before:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707_chassisBottom.jpg

Some more photos:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisback.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisInBetrieb1.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisInBetrieb2.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisInBetrieb3.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisInBetrieb4.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707chassisInBetrieb5.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Marconi707_mit_Bild.jpg

Kind regards

From Eckhard

I grew up with hard wiring b/f pcb construction but, after seeing that chassis I know a couple of things: 1) I'd hate to be an assembler, 2) I'd hete it even more to dig into it and do a lit of repairs and 3) I know think there is a reason that PCBs may be an improvement. Really nice to see a pre-war set operating again.

Hi Brian,

PCBs are easier to handle. But this hard wired chassis has its own beauty too. Especially I hate the repairing when I had to replace a cap in the farthest dark corner of the chassis. But nothing is impossible.

Here is a look at a nearly unmolested chassis:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/anotherChassis.jpg

Only the EHT caps, a few other caps and a potentiometer was replaced. But this photo is good for a comparison.

- Eckhard

Hi,

Today I have got the original Emiscope 3/2 for the Marconi 707. Some photos are here:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Emiscope3-2_1.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Emiscope3-2_2.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Emiscope3-2_3.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Emiscope3-2_4.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Emiscope3-2_5.jpg

The CRT looks like an Erlenmeyer flask. The EHT connection is in the socket. The picture tube has white getters, it is gassy. But I could not find broken glass. I am glad, that I have got the original CRT also, even it is not working.

Kind regards,
Eckhard

Eckhard, That Emiscope CRT is very interesting. Any chance you will let RACS try to rebuild it? Of is it too rare to risk ti?

John: I don't know how many of these Emiscope CRTs survive. If a safe rebuilding is possible, I would do it. But there are many unresolved problems.

Another problem is the aluminising of the phosphors to prevent ion burns. But a CRT with 2.5 KV EHT might be very dim with an aluminised screen? I am in contact with RACS in France.

When the CRT breaks during the rebuilding process, it is lost. No one would give me money for a broken crt.

Here is another interesting CRT:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/telefunken/img/RFB_VI-1_01.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/telefunken/img/RFB_VI-1_02.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/telefunken/img/RFB_VI-1_03.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/telefunken/img/RFB_VI-1_04.jpg

This is also a candidate for rebuilding. I had taken the photos with the mobil phone hence the low resolution. The accus of the camera were empty.

- Eckhard

Hi,

tonight I have tested the Emiscope 3/2 to see whether it has an ion burn. I could get a bright screen with only a light shadow of an ion burn. This might indicate that this CRT was not very often used for television.

The Emiscope and the Erlenmeyer flasks are made of the same material: Pyrex, Duranglas or borosilicate glass. Erlenmeyer flasks are very cheap. Would would it costs to built a new Emiscope 3/2 or 3/1 CRT, made out of an Erlenmeyer flask?

Kind regards,
Eckhard

Hi,

Steve Mcvoy has found a television picture tube for my Marconi set, a 7BP4. So, I could replace the radar tube with the 7BP4. Thanks to Steve for finding and sending it to me.

The 7BP4 is of the same design as the 7BP7, but with a different phosphor:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707with7BP4ATestcardC.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/707with7BP4AfullpictureDVB-t.jpg

Hugo Holden has made some suggestions for changes. I did

- short out N-81, but connected focus coil with 81.

- Attenuator added (R-C at the cathode of the CRT) due to the higher gain of the CRT.

- two 2M5 resistors were added parallel to R64 (bleeder chain) and CRT screen grid was connected with the middle point of both 2M5 resistors. I get 200V at screen grid, and when I switch off the set, the brightness on the screen disappears before the picture become a small spot which could damage the phosphor.

- I have added an internal vertical blanking with 10nF at the plate of the vertical output valve and 250K in series at the cathode of the picture tube. With this modification, I can lower the contrast and increase brightness without retrace lines on the screen.

- The X41C was weak. I have replaced the valve with a X41, one with a bakelite socket. This new valve provides more amplification than the valve before, I have to lower the R.F. input to -10 dB to get a clean picture. Otherwise I have moire and strong sound hum bars in the picture when contrast is fully turned on.

- I have replaced C64 with a non-electrolytic high grade film capacitor. I have kept the original cap on the chassis. Now I have no cracks and white spots in the picture. The original one has shot my U52.

Here are some recent photos of the chassis:

http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Marconi707with7BP4A_04.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Marconi707with7BP4A_03.jpg
http://bs.cyty.com/menschen/e-etzold/archiv/TV/marconi/img/Marconi707with7BP4A_05.jpg

Kind regards,
Eckhard